This application claims the priority of German patent document 198 44 239.4, filed Sep. 26, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for precise angle determination by means of a multiple-unit antenna radar system.
Several different methods are known for determining a target direction by means of radar. In the sequential lobing method (M. I. Skolnik, "Introduction to Radar Systems", McGraw-Hill 1980, Pages 153, 154), the echo signals in two antennas with different main beaming directions are superimposed, from which the precise direction to a target can be determined in a known manner. Although this method is very precise in the angular vicinity of the intersecting points of the antenna diagrams, major errors may occur in the angle range of the main beaming directions of the individual antennas, because of the form of the antenna diagrams.
The typical antenna diagrams with the pertaining discriminator curve are illustrated in FIG. 1. The upper portion of FIG. 1 is a graphic presentation of curves A1, A2 representing signal amplitude as a function of beaming angle .PHI. for each of the two participating antennas. (This schematic depiction illustrates only the major lobes without side lobes.) The lower portion of FIG. 1, on the other hand, shows the corresponding characteristic discriminator curve D1/2. In the sequential lobing method, the values of the characteristic curve are calculated from the amplitudes of the two antenna diagrams and are proportional to (A1-A2)/A1+A2) for a give angle .PHI..
As illustrated in FIG. 1, the characteristic discriminator curve is very steep in the range of .PHI.=0.degree., so that, in the intersecting points of the antenna diagrams, a very precise angle determination can be carried out. In contrast, it is flat in the range of the respective main beaming directions (maximal amplitude) of the two antennas so that the angle measurement for targets in this angle range can only be carried out imprecisely.
With such two-unit antenna systems, ambiguities may occur when several targets are present, because two similar targets, which each exist in the range of the main beaming directions of the antennas, may be protected by the system into the center (.PHI.=0). Inversely, a target may be divided from the center into two outer targets. In order to avoid this problem, particularly in the case of radar systems which are used on movable carriers for detecting the environment in the moving direction (such as motor vehicle ranging radar), frequently three-unit or five-unit antenna systems are used. In these systems, the main beaming direction of the center antenna points in the direction of movement.
The typical antenna diagrams A-1, A0, A1 of the participating antennas, together with the pertaining discriminator curves D0/-1, D0/1 in a three-unit antenna system are illustrated in FIG. 2, which demonstrates the disadvantage of systems with an odd number of antennas. The highest precision in the angle determination is achieved in the intersecting points of the antenna diagrams A-1, A0, A1, while, in the angle ranges of the main beaming directions of the antennas, because of the high directivity of the individual antennas (required for achieving a good angle resolution), the discriminator curve is very flat. This means that the angle error becomes relatively large there. In particular, the angle ranges with a flat discriminator curve include the main beaming direction of the radar system (.PHI.=0.degree.) which, in applications on movable carriers, frequently coincides with the direction of movement of the carrier.
An example of the technical construction of a multiple-unit antenna radar system, which can be used for the angle determination of a target, is illustrated in FIG. 3 (in this case, a three-unit antenna system). Radar systems according to FIG. 3 are known per se, and are used particularly in motor vehicle ranging radar systems. The illustrated radar system comprises three so-called feeds F-1, F0, F1 (also called feed points of the antenna) which can be individually controlled by way of an antenna change-over switch AU. The feeds are arranged to be spatially offset; in FIG. 3, for example, at different points of a straight line. The feeds illuminate a joint collimator, which here is constructed as a reflector R. (However, a lens, for example, may also be used.) Each of the respective individual antenna diagrams is produced by separate control of a different feed.
It is an object of the present invention to provide an angle determination method using a multiple-unit antenna radar system, that can achieve highly precise results, even in angle ranges in which only an indistinct measurement could be made heretofore.
Another object of the invention is to achieve such precision without installing additional antennas.
These and other objects and advantages are achieved by the method according to the invention, in which the individual antenna directions are generated by at least three feeds that are spatially offset with respect to one another and which illuminate a common collimator. With such a radar system, it is possible to produce several combination antenna diagrams by the simultaneous and phase-correct activation of at least two of the feeds; and a highly precise angle determination can be made by means of such combination antenna diagrams. According to the invention, combination antenna diagrams are generated so that their intersecting points are situated in angle ranges in which an exact angle determination is desired; particularly where the known separate control of the individual feeds achieves only an imprecise angle determination.
The method according to the invention is suitable for all multiple-unit antenna radar systems which have three or more spatially offset feeds. The number of feeds may be either odd or even; and the pertaining joint collimator may be constructed in particular as a lens or a reflector.
For actual angle determination on the basis of the generated combination antenna diagrams, the sequential lobing method can be used. However, other angle measuring processes which are known per se may also be used.
In many cases, it is especially important that a precise angle measurement be carried out in the main beaming direction of the multiple-unit antenna radar system (which in mobile applications, is frequently coincident with the direction of movement of the carrier). In order to achieve this precise angle measurement by controlling the feeds according to the invention, two combination antenna diagrams are constructed with their intersecting point situated precisely in the area of the main beaming direction of the radar system. Preferably, the combination antenna diagrams are symmetrical with respect to the main beaming direction of the radar system. Angle measurements can be made outside the main beaming direction by means of conventional methods; that is, the individual controlling of the feeds.
The method according to the invention has the advantage that additional precision can be achieved simply by a change in the control of the feeds. (A change of the hardware of the radar system is not required.) Therefore, a change-over can take place from the known measuring method (angle determination on the basis of the individual antenna diagrams) to the method according to the invention, so that the complete angle range can be covered with high precision, simply by means of appropriate software.
The method according to the invention can be used particularly in range warning radar systems on movable carriers, for example, in a motor vehicle, train or ship.